Human Genetics Section. The goal of this section is to determine the influence of genetic factors upon the expression of periodontal diseases and upon related etiologic and pathogenic factors. Preliminary analysis of 33 families of probands with juvenile periodontitis (JP) or severe periodontitis (SP) give strong evidence for a genetic component in the expression of JP that is probably autosomal recessive. In order to directly use complex segregation analysis rather than merely comparing the magnitudes of the likelihoods of competing genetic hypotheses, continued active recruitment of family members of the 131 probands (in 130 families) that we have identified is being undertaken, and new probands and their families are being sought. In addition to characterization of clinical traits of these subjects, blood samples are taken to determine the PMN chemotaxis responses of these individuals, since it has been established that PMN responses are abnormal in subjects with these diseases. Furthermore, antibody levels to putative bacterial pathogens in these diseases are being examined to determine whether there is a familial and/or genetic component evident in such responses.